Principal signs of inflammation include edema and bleeding, indicators of impaired vascular integrity. Vascular leakage often leads to impaired wound healing, and delayed clearance of pathogens, even hypotension and organ failure during severe systemic inflammation. Platelets prevent microvessels from leakage during inflammation. However, how platelets safeguard vascular integrity remains unclear. Our preliminary studies show that mice lacking platelet C-type lectin-like receptor 2 (CLEC-2) and its ligand podoplanin have increased vascular leakage in a model of lipopolysaccharide (LPS)-induced endotoxemia. These data support the novel hypothesis that interaction of platelet CLEC-2 with podoplanin protects vascular integrity in systemic inflammation. In this proposal, we will test this hypothesi by determining: 1) whether and if so, how platelet CLEC-2 interacts with perivascular podoplanin to protect vascular integrity during inflammation. Our preliminary data on confocal imaging detected staining for platelets on the abluminal side of venules and staining for podoplanin on perivascular macrophages during inflammation. These data suggest that interactions of CLEC-2 on extravasated platelets with podoplanin on perivascular macrophages limit vascular leakage during inflammation. In addition, platelets interact with leukocytes during inflammation. We will use spinning-disk and two-photon confocal intravital microscopy to determine whether platelets emigrate with or follow emigrating leukocytes through venules to interact with perivascular podoplanin. Models of systemic inflammation (such as cecal ligation and puncture) will be used to determine whether platelet CLEC-2 and perivascular podoplanin protect vascular integrity in different organs. 2) How platelets activated by podoplanin maintain vascular integrity during inflammation. We hypothesize that podoplanin-CLEC-2 interactions induce local platelet secretion of S1P that activates S1PR1 on endothelial cells to stabilize endothelial adherens junctions during inflammation. We will examine whether deficient podoplanin-CLEC-2 interactions exacerbate or platelet S1P ameliorates vascular leakage and organ failure in systemic inflammation. We will determine whether platelet S1P protects vascular integrity by maintaining endothelial adherens junctions during inflammation. Our preliminary data show that platelets, after podoplanin-CLEC-2-mediated activation, also release angiopoietin 1 (Ang1), another mediator of vascular integrity. Therefore, we will determine whether platelet S1P and Ang1 function cooperatively to protect vascular integrity during inflammation. These studies will provide key mechanistic insights into how platelet CLEC-2 limits vascular permeability during inflammation. Our results could offer exciting translational applications, for example, S1P receptor agonists or related drugs to restore endothelial barrier function in inflammatory diseases such as sepsis.